Spherical WC-Co Powder Prepared By Radio Frequency Plasma And Its Microstructure And Properties For 3D Printing

Cemented carbide has the characteristics of high hardness,good strength and toughness,as well as wear resistance,heat resistance and corrosion resistance.It is widely used in military industry,national defense,geological prospecting and industrial production.Traditional powder metallurgy methods are difficult to prepare complex-shaped cemented carbide parts and tools,which limits the excellent performance of cemented carbide.Metal 3D printing technology can directly realize the rapid manufacturing of complex-shaped cemented carbide parts.3D printing requires powders with high parament density and high sphericity as raw materials.The cemented carbide powders prepared by the current commercial spray granulation and sintering crushing methods cannot meet the requirements of both aspects at the same time.Radio frequency(RF)plasma has the advantages of high energy density,high plasma torch temperature,no electrode contamination and controllable reaction atmosphere.It is one of the most effective methods for preparing high-quality spherical powders with uniform composition,high sphericity,and good flowability,especially suitable for preparing refractory metal spherical powder.This article aims to develop high-quality spherical cemented carbide powders for3D printing.Using spray granulated WC-30Co powder as raw material,the influence of radio frequency plasma spheroidization process and subsequent heat treatment on the structure and properties of the powder is focused on.The effect of selective electron beam melting(SEBM)of the powder and the influence of hot isostatic pressing on the performance of the formed part are also investigated.The research content is as follows:(1)Using spray granulated powder as raw material,spherical WC-Co powder was prepared by radio frequency plasma spheroidization,and the effects of spheroidization process parameters and subsequent heat treatment on the microstructure and properties of the powder were studied.The spheroidized powder particles have a smooth surface,high sphericity,and compact interiors.The spheroidizing treatment significantly improves the apparent density,tap density and flowability of the powder.The process parameters in the spheroidizing process have an important influence on the spheroidizing effect.As far as the composition of the sheath gas is concerned,as the hydrogen added to the sheath gas increases from 0L/min to 6L/min,the thermal conductivity of the plasma torch gas gradually increases,and the“Ferrero”particles with rough surfaces in the spheroidized powder decrease.The spheroidizing effect of the powder is obviously improved.When the hydrogen flow rate is increased to 6 L/min,the powder feeding rate changes within a large range from 40.5 to 81 g/min,and 100%spheroidization can be achieved.However,with the decrease of the powder feeding rate and the increase of the hydrogen flow rate in the sheath gas,the Co content of the spheroidized powder decreases,and the phase composition of the spheroidized powder changes,resulting in harmful phases such as W₂C,Co₃W₃C and free carbon.After subsequent vacuum heat treatment at 950?,the phase composition of the powder can be transformed into WC and Co,and the free carbon content is controlled at a reasonable level(0.06 wt.%).The heat-treated powder still maintains good sphericity and can be used as raw material powder for 3D printing.(2)The effect of SEBM process parameters on the porosity and microstructure of the formed part was studied.An increase in the beam current or a decrease in the scan rate can reduce the porosity of the print.At the same energy density,the porosity of the print decreases with the increase of the scan rate.The minimum porosity of the formed part is0.22%.WC grains grow rapidly during the SEBM printing process,and a layered structure of staggered WC grains of different sizes appears on the vertical section,but there is no such structure on the horizontal section.The Vickers hardness of the horizontal section of the formed part is slightly higher than the Vickers hardness of the vertical section.(3)The effect of hot isostatic pressing(HIP)treatment on the microstructure and properties of SEBM formed parts was studied.HIP can significantly eliminate pores in SEBM prints and increase the density of the sample.After HIP treatment,the phase composition of the sample did not change,and it was still WC and Co.In the vertical section of the sample,there is still a layered structure with different sizes of WC grains staggered,and there are Co pools that cannot be eliminated by hot isostatic pressing.After hot isostatic pressing,the Vickers hardness of the horizontal section and the vertical section showed a certain uniformity.